Fairly common in grands, I've seen these used in the wippens (usually as a component used for adjustment on the repetition lever). 5/16" diameter, about .120" thick, woven green felt.  A little too small for let off punchings which are usually 3/8" or 7/16" diameter and are available in several thicknesses in my eBay store.   

Free Tech support! I've been a piano & player piano technician for 30+ years,  I'm glad to answer any questions about this item before or after the sale - my contact info is included with each shipment and you can also reach me through eBay.